1. Technical Field of the Invention
The present invention relates to a microelectromechanical modulation device which has a movable portion that is bi-directionally displaced, to a microelectromechanical modulation device array, and to an image forming apparatus. More particularly, the present invention relates to improved techniques for damping the movable portion.
2. Description of the Related Art
In recent years, rapid progress of MEMS (MicroElectroMechanical Systems) technology has resulted in extensive development of a microelectromechanical modulation device that electrically displaces or moves a microstructure having a size in the order of micrometers. This microelectromechanical modulation device is, for example, a digital micromirror device (DMD), which deflects light by inclining a micromirror. The DMD is used for various purposes, such as a projection display, a video monitor, a graphic monitor, a television set, and an electrophotographic printing machine, in the field of optical information processing.
Generally, the microelectromechanical modulation device has a movable portion that is elastically displaceably supported and is bidirectionally displaced. This movable portion mainly performs a modulation operation. Therefore, it is very important for favorably performing a switching operation to control the damping of the movable portion.
For example, a micromirror device disclosed in JP-A-8-334709 (hereinafter referred to as JPA'709) is configured so that a voltage is applied to one of a pair of drive electrodes, and that a movable portion having the mirror difsposed between these electrodes is rotated by an electrostatic attracting force determined according to the difference in electrical potential between the movable portion and the drive electrode and to the electrostatic capacity therebetween.
Further, a method of damping ribbon devices in a microelectromechanical grating device disclosed in JP-A-2001-174720 (hereinafter referred to as JPA'720) is a method of damping electromechanical ribbon devices over a channel, which defines a bottom surface and has a bottom conductive layer formed below this bottom surface. This method includes the step of providing at least one constant amplitude voltage pulse to at least one ribbon device and the step of providing at least one damping pulse, which is separated by a narrow tentative gap from the constant amplitude voltage pulse, to the ribbon devices. That is, an electrostatic force is caused by one movable portion electrode and one fixed electrode of a parallel plate type device to act in a single direction. Further, the oscillation of the ribbon devices is controlled by a driving voltage, which used for attracting the ribbon device to a lower electrode, and two damping drive voltages that include an initial damping voltage, which is applied immediately before the application of the driving voltage, and a final damping voltage that is applied immediately after the application of the driving voltage.
Furthermore, an optical path switching device disclosed in JP-A-2002-169109 (hereinafter referred to as JPA'109) has a mechanical optical switch, which switches an optical path by applying a signal voltage to an electromagnetically driven actuator, and a control circuit that supplies a signal voltage to the optical switch. Regarding the signal voltage, let VH and T denote a rise amplitude and a width of a signal, respectively. When a time (T/2) elapses since the rise of the signal, the voltage level of the signal is equal or less than (⅔)VH. Further, when a time (T/2) elapses since the rise of the signal having a width T, the signal voltage applied to the actuator is reduced to a value that is equal to or less than (⅔) times the rise amplitude to thereby suppress the oscillation of a movable portion.
Additionally, a method of controlling a micromachine device, which is disclosed in JP-A-2002-36197 (hereinafter referred to as JPA'197), is such that a first control signal and a second control signal are supplied to the micromachine device, that the second control signal sets the micromachine device to be in an active state, and that the first control signal causes this state of the micromachine device to maintain this state. The micromachine device is controlled by using at least two control signals, one of which sets the micromachine device to be in a pull-in state, and the other of which causes the micromachine device to maintain the pull-in state thereof. This enables the control of the micromachine device at a low voltage level.
However, in the micromirror device disclosed in JPA'709, a voltage is applied to one of the drive electrodes. An electrostatic attracting force is generated according to the difference in potential between the movable portion and the drive electrode and the electrostatic capacity therebetween. Thus, the movable portion is rotated. Consequently, as illustrated in FIG. 21A, a micromirror is transited to a contact position by the application of a voltage Va to the drive electrode. Immediately after the micromirror reaches the contact position, the mirror receives a reaction force from a contact member. Thus, the oscillation of the movable portion occurs. Even in a noncontact structure in which the micromirror does not reach the contact position, as illustrated in FIG. 21B, the movable portion passes over a desired angle (that is, a convergence position), so that an overshoot of the mirror occurs. Consequently, it takes time to stop the oscillation. Such oscillation and overshoots hinder the speeding-up of a switching operation of the microelectromechanical modulation device.
Further, in the case of the micromechanical grating device disclosed in JPA'720, as illustrated in FIG. 22A, the constant amplitude voltage pulse 1 is a function of time. The constant amplitude voltage pulse 1 has a duration of 2 μseconds and also has a voltage value of 10V. Just after the constant amplitude voltage pulse 1, a narrow damping pulse 5 separated from the constant amplitude voltage pulse 1 by a narrow gap 3 is applied. Furthermore, as illustrated in FIG. 22B, in a case where an adjacent contact amplitude voltage pulse 7 has an opposite polarity, the polarity of a damping pulse 9 is opposite to that of the associated voltage pulse 7. However, this microelectromechanical grating device is what is called the parallel plate type microelectromechanical modulation device configured so that a ribbon serving as a movable portion is parallel displaced to a substrate, and that thus, damping is performed by applying pulses to one movable-portion-side electrode and one fixed side electrode facing this movable-portion-side electrode. Consequently, this device has a disadvantage in that this device is poor in diversity of oscillation control methods. For instance, when the movable portion is attracted and displaced to the substrate, an opposite damping force cannot be applied to the movable portion. That is, the oscillation thereof cannot be actively reduced.
Moreover, the optical path switching device disclosed in JPA'109 is adapted so that when the movable portion approaches an end of a yoke in the electromagnetically-driven actuator, that is, when the attracting force due to the magnitude of a magnetic field of the permanent magnet is enhanced, the movable portion is moved to a fiber-connection position so as to reduce the magnitude of the attracting force due to a coil magnetic field to thereby prevent the magnitude of a total attracting force from becoming too high. A signal outputted from a signal generation circuit has a waveform representing a signal voltage whose rise voltage VH is 7V and whose level drastically drops after the rise. The width T of the signal is 5 ms. The voltage at an end of the signal is 0.5V. When a time (T/2) elapses since the rise of the signal, the voltage level of the signal is 2.8V. In a case illustrated in FIG. 23B, the rise voltage is 7V. The width T of the signal is 5 ms. A time TO, in which the amplitude changes like a step, is 1.5 ms. In a case illustrated in FIG. 23C, the rise voltage is 5V. A time T′, which is taken until the reduced amplitude reaches 1V, is 2 ms. The time T′ corresponds to the width T of a signal. The application of the voltage of 1V is continued until the next switching is performed after the lapse of the time T′. In a case illustrated in FIG. 23D, the rise voltage is 5V. The time TO, in which the amplitude changes like a step, is 3 ms. A waveform is changed like a step, so that the amplitude is reduced to a constant value of 0.5V. The rise amplitudes of the waveforms representing these signal voltages are set to be large thereby to speed up the movement of the movable portion (that is, increase the switching rate thereof). When the movable portion starts to move, the signal voltage is rapidly lowered to thereby reduce the magnitude of the force applied to the movable portion. Thus, chattering can be suppressed. However, this optical path switching device is adapted so that the a block serving as the movable portion is bidirectionally parallel displaced, and that the oscillation of the movable portion is suppressed by changing a driving force acting in a forward direction. Consequently, this optical path switching device has a disadvantage in that this device is poor in diversity of oscillation control methods. Also, this optical path switching device is adapted so that basically, the magnitude of an attracting force due to the coil magnetic field is reduced, and that the signal voltage is reduced so as to prevent the magnitude of a total attracting force from being too high. Thus, similarly to the microelectromechanical grating device, this optical path switching device cannot actively reduce the oscillation of the movable portion.
Furthermore, according to the method of controlling a micromachine device, which is disclosed in JPA'197, the micromachine device is controlled by using a single or a plurality of control signals. FIGS. 24A to 24H show exemplary waveforms of this control signal. As is seen from FIGS. 24A and 24b, the control signal may be a pulse train that changes the state of the micromachine device. Similarly, in the case of using at least two control signals, these signals may be a signal synthesized from superimposed signals respectively shown illustrated in FIGS. 22C and 22D, an amplitude modulated (AM) signal illustrated in FIG. 24E, a frequency modulated (FM) signal illustrated in FIG. 24F, a pulse width modulated (PWM) signal, and a pulse density modulated (PDM) signal illustrated in FIG. 24H. However, this control method aims at reduction of a holding voltage in the pull-in state, reduction of an on/off delay due to discharge of residual electric charges, and increase in amplitude of an output signal. Thus, this control method cannot actively reduce the oscillation of the movable portion.